Beneath the rural tranquillity of the Geneva countryside, where ramshackle sheds dot the wide-open fields, scientists are getting ready for a trip into the unknown. Here, under 100 metres of rock and sandstone, lies the biggest, most complex machine humans have ever built, and on Wednesday they will finally get to turn it on.

For Cern, the European nuclear research organisation, it will mark the end of a lengthy wait and the beginning of a new era of physics. Over the next 20 years or so, the $9bn (£5bn) machine will direct its formidable power towards some of the most enduring mysteries of the universe.

The machine will search for extra dimensions, which could be curled up into microscopic loops. It might produce "dark matter", the unknown substance that stretches through space like an invisible skeleton. And it will almost certainly discover the elusive Higgs boson, which helps explain the origin of mass, and is better known by its wince-inducing monicker, the God particle.

At least that is the hope. For the machine to work a dizzying number of electronic circuits, computer-controlled valves, airtight seals and superconducting magnets must all work in concert.

The machine is called the Large Hadron Collider (LHC), and when working at full tilt it will drive two beams of particles in opposite directions around a 17 mile (27km) ring at 99.9999991% of the speed of light. Every second each of the beams will complete 11,245 laps of the machine.

At four points around the ring the beams will be steered into head-on collisions, causing the particles to slam into one another with enough energy to recreate in a microcosm the violent fireball conditions that existed one trillionth of a second after the big bang. Giant detectors, one of which is so enormous it sits in a cavern that could accommodate the nave of Westminster Abbey, will then scrutinise the shower of subatomic debris in the hope of finding something no one has ever seen before.

"This is a once in a generation kind of machine, and we really don't know what we will find," said Brian Cox, a physicist at Manchester University who works on Atlas, LHC's largest detector. "It's like going to Mars. You know you're going to find something new, because you're going where no one has been before."

The awesome power of the LHC has prompted a flurry of alarmist fears that the machine might create a black hole that would swiftly consume the planet. In the run-up to the machine switching on the laboratory has received a steady stream of calls from people wanting reassurance, or simply asking the scientists to stop. Two attempts to stop the machine through the courts were dismissed. Scientists say far more energetic collisions happen regularly in nature, when cosmic rays strike stray particles in space.

The project has drawn more down-to-Earth criticisms too. Sir David King, the government's former chief science adviser, believes it diverts top scientists away from tackling the more pressing issues of the time, such as climate change and how to decarbonise the economy. In total Britain has contributed more than £500m towards the LHC project.

Although Cern has already conducted some basic tests with its machine, Wednesday will be the first attempt to get a beam of subatomic particles called protons circulating inside it.

"If the beam goes all the way round on the first go, that would be quite amazing. It's never happened in the history of particle colliders," said Cern's James Gillies. If the test is successful, scientists may try to send the beam around in the opposite direction, though first collisions are not expected until next month.

They expect to spend a few months getting to grips with the machine before putting it to work in earnest. "People might think we already know a lot about the way things work, but the wheels are coming off our understanding of the universe. We can confidently say that 95% of the universe is made up of stuff we don't understand," said Cox.

Only 5% of the universe is made of matter scientists understand. A further 25% is so-called "dark matter", which clusters around galaxies, and the remaining 70% is even more enigmatic "dark energy", which drives the expansion of the universe.

One of the first discoveries that could emerge is proof of a theory known as supersymmetry. According to the theory, every particle in the universe has a slightly overweight but invisible twin. One of these, called the neutralino, is a leading candidate for dark matter, and could be made as soon as the machine performs its first collisions.

For the 10,000 scientists and engineers involved in the project this is the culmination of more than 20 years of work, the last eight of which were dedicated to bolting the machine together. The machine is designed to reach energies seven times higher than the existing most powerful particle collider in the world, the Tevatron at Fermilab near Chicago.